Metallic compositions useful for brazing, and related processes and devices

ABSTRACT

A braze alloy composition is disclosed, containing nickel, about 5% to about 40% of at least one refractory metal selected from niobium, tantalum, or molybdenum; about 2% to about 32% chromium; and about 0.5% to about 10% of at least one active metal element. An electrochemical cell that includes two components joined to each other by such a braze composition is also described. A method for joining components such as those within an electrochemical cell is also described. The method includes the step of introducing a braze alloy composition between a first component and a second component to be joined, to form a brazing structure. In many instances, one component is formed of a ceramic, while the other is formed of a metal or metal alloy.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional of U.S. patent application Ser. No.13/628,548, filed on Sep. 27, 2012, and claims priority therefrom.

TECHNICAL FIELD

This invention generally relates to a braze composition. In some specific embodiments, the invention relates to a braze composition that provides corrosion-resistant sealing and other benefits to components used at high temperatures, e.g., thermal rechargeable batteries.

BACKGROUND OF THE INVENTION

A variety of electrochemical devices require processes and compositions for providing seals on or within the devices. The seals may be used to encapsulate the entire device, or they may separate various chambers within the device. As an example, many types of seal materials have been considered for use in high-temperature rechargeable batteries/cells for joining different components.

Sodium/sulfur or sodium/metal halide are good examples of high-temperature batteries that may include a variety of ceramic and metal components. The ceramic components often include an electrically insulating alpha-alumina collar, and an ion-conductive electrolyte beta-alumina tube, and are generally joined or bonded via a sealing glass. The metal components usually include a metallic casing, current collector components, and other metallic components which are often joined by welding or thermal compression bonding (TCB). While mechanisms for sealing these components are currently available, their use can sometimes present some difficulty. For example, metal-to-ceramic bonding can be challenging, due to thermal stress caused by a mismatch in the coefficient of thermal expansion for the ceramic and metal components.

The metal-to ceramic bonding is most critical for the reliability and safety of the high-temperature cells. Many types of seal materials and sealing processes have been considered for joining metal to ceramic components, including ceramic adhesives, brazing, and sintering. However, most of the seals may not be able to withstand high temperatures and corrosive environments.

A common bonding technique for joining ceramic and metal components involves multiple steps of metalizing the ceramic component, followed by bonding the metallized ceramic component to the metal component using a thermal compression bond (TCB). The bond strength of such metal-to-ceramic joints is controlled by a wide range of variables. Some of the variables include the microstructure of the ceramic component, the metallization of the ceramic component, and various TCB process parameters. In order to ensure good bond strength, the process requires close control of several parameters involved in various process steps. In short, the method is relatively expensive, and complicated, in view of the multiple processing steps, and the difficulty in controlling the processing steps.

Brazing is another potential technique for making the ceramic-to-metal joints. A braze material is heated above its melting point, and distributed between two or more close-fitting parts by capillary action. However, most of the brazing materials (or braze materials) have limitations that prevent them from fulfilling all of the necessary requirements of high temperature batteries. Moreover, some of the commercial braze materials can be quite expensive themselves; and using them efficiently in various processes can also be costly. Nonetheless, brazing techniques remain of considerable interest for joining ceramic and metallic parts in various high-temperature devices.

In view of some of these concerns and challenges, it may be desirable to develop new braze alloy compositions that have properties and characteristics that meet performance requirements for high temperature rechargeable batteries, and are less complicated and less expensive to process, as compared to the existing sealing methods.

BRIEF DESCRIPTION

An embodiment of this invention is directed to a braze alloy composition, comprising:

-   -   a) nickel;     -   b) about 5% to about 40% of a refractory metal selected from         niobium, tantalum, molybdenum, or combinations thereof;     -   c) about 2% to about 32% chromium; and     -   d) about 0.5% to about 10% (total) of at least one active metal         element, based on the total weight of the composition.

Another embodiment of the invention is directed to an electrochemical cell, comprising a first component and a second component joined to each other by a braze alloy composition as described above.

A method for joining components forms the basis for another embodiment of this invention. The method comprises the step of introducing a braze alloy composition between a first component and a second component to be joined, to form a brazing structure. The braze alloy composition is as mentioned above, and further described in the remainder of this disclosure. In this method, the brazing structure that is put into place is heated to form an active braze seal (joint) between the first component and the second component.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a cross-section of an electrochemical cell, according to some embodiments of this invention.

FIG. 2 is a depiction of a scanning electron micrograph of a cross-section of a brazed joint between a ceramic component and a metal component.

DETAILED DESCRIPTION

The invention includes embodiments that relate to a braze alloy composition for providing various types of seals. Non-limiting examples include the seals that are required in various electrochemical cells, e.g., those in a sodium/sulfur or a sodium metal halide battery. The invention also includes embodiments that relate to devices made by using the braze composition. As discussed in detail below, some of the embodiments of the present invention provide a braze alloy for sealing a ceramic component to a metal component, e.g., in an electrochemical cell; along with a metal halide battery formed thereof. These embodiments advantageously provide an improved seal and method for the sealing. Although the present discussion provides examples in the context of a metal halide battery, these processes can be applied to any other application, including ceramic-to-metal or ceramic-to-ceramic joining.

When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements, unless otherwise indicated. The terms “comprising,” “including,” and “having” are intended to be inclusive, and mean that there may be additional elements other than the listed elements. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Unless otherwise indicated herein, the terms “disposed on”, “deposited on” or “disposed between” refer to both direct contact between layers, objects, and the like, or indirect contact, e.g., having intervening layers therebetween.

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it may be related. Accordingly, a value modified by a term such as “about” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.

As used herein, the term “liquidus temperature” generally refers to a temperature at which an alloy is transformed from a solid into a molten or viscous state. The liquidus temperature specifies the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium. Above the liquidus temperature, the alloy is homogeneous, and below the liquidus temperature, an increasing number of crystals begin to form in the melt with time, depending on the particular alloy. Generally, an alloy, at its liquidus temperature, melts and forms a seal between two components to be joined.

The liquidus temperature can be contrasted with a “solidus temperature”. The solidus temperature quantifies the point at which a material completely solidifies (crystallizes). The liquidus and solidus temperatures do not necessarily align or overlap. If a gap exists between the liquidus and solidus temperatures, then within that gap, the material consists of solid and liquid phases simultaneously (like a “slurry”).

“Sealing” is a function performed by a structure that joins other structures together, to reduce or prevent leakage through the joint between the other structures. The seal structure may also be referred to as a “seal” or “joint” herein, for the sake of simplicity.

Typically, “brazing” uses a braze material (usually an alloy) having a lower liquidus temperature than the melting points of the components (i.e. their materials) to be joined. The braze material is brought slightly above its melting (or liquidus) temperature while protected by a suitable atmosphere. The braze material then flows over the components (known as wetting), and is then cooled to join the components together. As used herein, “braze alloy composition” or “braze alloy”, “braze material” or “brazing alloy”, refers to a composition that has the ability to wet the components to be joined, and to seal them. A braze alloy, for a particular application, should withstand the service conditions required, and should melt at a lower temperature than the base materials; or should melt at a very specific temperature. Conventional braze alloys usually do not wet ceramic surfaces sufficiently to form a strong bond at the interface of a joint. In addition, the alloys may be prone to sodium and halide corrosion.

As used herein, the term “brazing temperature” refers to a temperature to which a brazing structure is heated to enable a braze alloy to wet the components to be joined, and to form a braze joint or seal. The brazing temperature is often higher than or equal to the liquidus temperature of the braze alloy. In addition, the brazing temperature should be lower than the temperature at which the components to be joined may become chemically, compositionally, and mechanically unstable. There may be several other factors that influence the brazing temperature selection, as those skilled in the art understand.

Embodiments of the present invention provide a braze alloy composition capable of forming a joint by “active brazing” (described below). In some specific embodiments, the composition also has high resistance to sodium and halide corrosion. The braze alloy composition includes nickel, at least one selected refractory metal, chromium, and at least one active metal element, as described herein. Each of the elements of the alloy usually contributes and optimizes at least one property of the overall braze composition.

These properties may include liquidus temperature, coefficient of thermal expansion, flowability or wettability of the braze alloy with a ceramic; corrosion resistance, and ease-of-processing. Some of the properties are described below.

According to most of the embodiments of the invention, the braze alloy composition is a nickel-based alloy. In other words, the alloy usually contains a relatively high amount of nickel, as compared to the amount of other elements in the alloy. Nickel is relatively inert in a corrosive environment, as compared to other known base metals, e.g. copper, iron, chromium, cobalt, etc. Additionally, it is observed that nickel may enhance other properties of the braze alloy, such as the thermal expansion coefficient, and the phase stability.

In some embodiments of this invention, a suitable level for the amount of nickel may be at least about 30%, based on the total weight of the braze alloy. Very often, nickel is present in an amount of at least about 45%. In some embodiments that are preferred for selective end-use applications, the nickel is present from about 50% to about 70%, based on the total weight of the braze alloy, and more often, from about 50% to about 65%.

As mentioned above, the concept of “active brazing” is important for embodiments of this invention. Active brazing is a technique often used to join a ceramic to a metal, or a ceramic to a ceramic. Active brazing uses an active metal element that promotes wetting of a ceramic surface, enhancing the capability of providing a hermetic seal. An “active metal element”, as used herein, refers to a reactive metal that has high affinity to the oxygen within the ceramic, and thereby reacts with the ceramic. A braze alloy containing an active metal element can also be referred to as an “active braze alloy.” The active metal element undergoes a reaction with the ceramic, when the braze alloy is in a molten state, and leads to the formation of a thin reaction layer at the interface of the ceramic and the braze alloy. The thin reaction layer allows the braze alloy to wet the ceramic surface, resulting in the formation of a ceramic-ceramic or a ceramic-metal joint/bond, which may also be referred to as an “active braze seal.”

Thus, an active metal element is an essential constituent of a braze alloy for employing active brazing. A variety of suitable active metal elements may be used to form the active braze alloy. The selection of a suitable active metal element mainly depends on the chemical reaction with the ceramic (e.g., alumina) to form a uniform and continuous reaction layer, and the capability of the active metal element (e.g., as measured by the Gibbs free energy of formation) to form an alloy with a base alloy. (In this instance, the base alloy is nickel with chromium and selected refractory elements, as discussed below).

In terms of cost, availability, and performance, the active metal element for embodiments herein is often titanium. However, for other embodiments, zirconium is preferred; and in some cases, hafnium is preferred. Under certain conditions and for different types of “mating” surfaces, each of these elements might be especially suitable for diffusing into and reacting with a ceramic surface during brazing. A continuous transitional layer (i.e., the “reaction layer”) provides a wettable surface that has a semi-metallic character. In this manner, a coherent braze joint is formed between the components. In other embodiments, it may sometimes be advantageous to include vanadium as the active metal.

The presence and the amount of the active metal may influence the thickness and the quality of the thin reaction layer, which contributes to the wettability or flowability of the braze alloy, and therefore, the bond strength of the resulting joint. In some embodiments, the active metal is present in an amount no greater than about 10 weight percent, based on the total weight of the braze alloy. A suitable range is often from about 0.5 weight percent to about 5 weight percent. In some specific embodiments (though not all), the active metal is present in an amount ranging from about 1 weight percent to about 3 weight percent, based on the total weight of the braze alloy. The active metal element is generally present in small amounts suitable for improving the wetting of the ceramic surface, and forming the thin reaction layer, for example, less than about 10 microns. A high amount of the active metal layer may cause or accelerate halide corrosion.

The braze alloy composition of this invention further comprises a refractory element selected from niobium, tantalum, and combinations thereof. The refractory element is especially useful for providing strength and high-temperature resistance to the braze. A refractory element like niobium can also provide good corrosion-resistance in a sodium-containing environment. Moreover, the refractory element, along with selected amounts of nickel and chromium (discussed below), effectively forms a ternary alloy that provides the overall braze composition with a liquidus temperature below about 1350° C. (In most embodiments, the braze alloy has a liquidus temperature lower than the melting temperatures of the components being joined by the braze.)

The liquidus temperature is an important feature for the braze alloy, in terms of its flow properties and wetting capabilities. As described below, these properties are especially critical in the sealing of metal-ceramic components (e.g., ring-collar sealing) within a high-temperature battery. In some preferred embodiments, the refractory element(s), nickel, and chromium are present in ratios that provide the overall braze composition with a liquidus temperature less than about 1250° C.

In many specific embodiments, the refractory element is niobium (by itself), or a refractory composition that contains at least about 50% niobium, by weight, e.g. with the balance comprising tantalum. When niobium is the refractory element, it is usually present at a level in the range of about 5% to about 20%, based on the total weight of the braze composition. In some preferred embodiments, the level is in the range of about 10% to about 15%. (Specific levels also depend on the relative levels of the active metal and chromium as well). However, in other instances, the level of niobium may extend up to about 30% by weight, and in some instances, up to about 40% by weight. It should be noted, though, that the presence of relatively high levels of niobium can in some cases result in the formation of brittle intermetallic phases, so very often, the lower levels of niobium are preferred, within the ranges set forth above.

In other embodiments, the refractory element is tantalum. In braze compositions for various embodiments, tantalum is usually present at a level in the range of about 5% to about 25%, based on the total weight of the braze composition. As in the case of niobium, there may be applications where the level of tantalum may extend up to about 30% by weight, and in some instances, up to about 40% by weight. However, the relatively high levels of tantalum may result in an alloy with a liquidus temperature beyond about 1350° C.-1400° C., thereby making many braze applications (though not all applications) impractical. In some specific embodiments, the level of tantalum is in the range of about 5% to about 20%, and preferably, in the range of about 10% to about 20%.

As alluded to previously, a niobium-tantalum combination is also possible. The ratio (Nb to Ta) of the two elements could be in the range of about 3:1 to about 1:3. (The specific proportions of each element will also depend on the desired liquidus temperature, as mentioned above).

In some end use applications, the refractory element may be molybdenum, alone or in combination with other refractory elements. The use of molybdenum may result in a relatively high liquidus temperature for the braze composition. However, if a component being brazed is formed from molybdenum, higher brazing temperatures may be called for, as compared to brazing nickel. For example, the metal rings used in sealing systems for batteries, described below, could possibly be formed of molybdenum or a molybdenum alloy. In those instances, a molybdenum-containing braze may be very appropriate. The level of molybdenum will vary, based on the general factors discussed herein (such as melting temperature). Usually, the various ranges described above for tantalum would also be appropriate for molybdenum.

Chromium is another important constituent for the braze alloy composition. Chromium plays a key role in environmental resistance, e.g., resistance to “hot corrosion”, mixed-gas attack, and mechanical damage, like erosion. Chromium can also be important for enhancing the high temperature strength of the braze, and its inherent oxidation resistance.

The level of chromium present is based on a number of factors, including the environment in which the braze material will be employed, as well as the relative amounts of nickel and the refractory element(s) that are present. Usually, the level of chromium is about 2% to about 32%, based on the weight of the braze composition. In some specific embodiments, the level is in the range of about 10% to about 30%. In some especially preferred embodiments—especially when joining components within a sodium-metal halide thermal battery, the level of chromium is in the range of about 25% to about 30%.

In some embodiments (though not all), the braze alloys described herein may also include cobalt. The addition of cobalt can further enhance the corrosion resistance of the overall composition. Cobalt is usually present in relatively small amounts, e.g,. about 0.5% to about 20% by weight. In some preferred embodiments, the level is about 5% to about 10%.

Another optional constituent is palladium. In the case of sodium metal halide electrochemical cells, the presence of palladium can further enhance corrosion resistance in the sodium-containing environment. In other end use applications, palladium can function as a melting point depressant. The melting point depressant can decrease the viscosity of the molten alloy, and in turn, increase its “flowability” or wettability. In some embodiments, the braze alloy includes up to about 10 weight percent palladium (e.g., about 0.5 weight percent to about 10 weight percent), based on the total weight of the alloy.

In the case of some of the thermal battery applications, the particular nature of the electrode and electrolyte compositions, and their chemical reactions, can influence the inclusion or exclusion of elements in the braze composition which may sometimes interact with the battery chemistry. One example is provided in the case of sodium metal halide electrochemical cells. Aluminum is believed to be chemically stable in the secondary electrolyte of the cell, typically NaAlCl₄, and can sometimes be included in the active braze compositions, usually at a level less than about 5% by weight (e.g., 0.5% by weight to about 5% by weight). However, in other situations for these types of cells, aluminum may adversely react with additives that may be used in the cathode of the cell, and should therefore be omitted entirely.

Another example relates to iron, which can also be an important constituent in sodium metal halide chemistry, i.e., in the electrode activity of the cell. In general, iron is chemically stable in both the cathodic and anodic environment of the cell. However, iron can become electrochemically active at the cell's operating voltages, and this can be problematic, especially when the cells need to be filled almost entirely with electrochemical components, for greater energy density. While braze sealing mechanisms for joining ceramic-metal components in the cell do not participate electrochemically, the presence of iron in the braze may result in the braze itself becoming electrochemically active, and this could lead to a decrease in braze integrity. Thus, in some preferred embodiments, the braze composition must be free of any iron.

Gold and silver are ductile precious metals that can also reduce the liquidus temperature and, thus, lower the brazing temperature. However, their presence can sometimes be problematic in the case of sodium metal halide electrochemical cells. These metals tend to form various intermetallics with sodium, at the operating temperature of the cells, and this can promote corrosion when the cell is in operation. Thus, it is often preferred that gold and silver, if present, each be at a level no greater than about 10% by weight. In some specific embodiments, the braze composition should be free of each of these metals.

As mentioned previously, other embodiments of this invention are directed to an electrochemical cell that comprises a first component and a second component joined to each other by a braze alloy composition. The cell may be a sodium-sulfur cell or a sodium-metal halide cell, for example. The braze alloy composition is as described above, and comprises nickel, at least one refractory element, chromium, and at least one active metal. The respective amounts of the alloy constituents are described above. In some embodiments, the braze alloy composition consists essentially of nickel, the refractory metal(s), chromium, and at least one active metal element. In other embodiments, the braze alloy composition further consists essentially of at least one of palladium or cobalt. (Those skilled in the art understand that trace amounts of various elements, e.g., at impurity levels, can be introduced into an alloy from various sources, during preparation and use. These trace amounts can generally be discounted as insignificant).

As also discussed above, the first component of the electrochemical cell often comprises a metal or a metal alloy, and the second component often comprises a ceramic. The metal component can be a ring formed of a variety of materials, such as nickel, niobium, molybdenum, nickel-cobalt ferrous alloys (e.g., Kovar™ alloys), and the like. The ceramic component can be a collar that includes an electrically insulating material, such as alumina. One specific illustration of such a cell, containing metal-to-ceramic joints, is provided in FIG. 1.

FIG. 1 is a schematic diagram depicting an exemplary embodiment of a sodium-metal halide battery cell 10. The cell 10 has an ion-conductive separator tube 20 disposed in a cell case 30. The separator tube 20 is usually made of β-alumina or β″-alumina. The tube 20 defines an anodic chamber 40 between the cell case 30 and the tube 20, and a cathodic chamber 50, inside the tube 30. The anodic chamber 40 is usually filled with an anodic material 45, e.g. sodium. The cathodic chamber 50 contains a cathode material 55 (e.g. nickel and sodium chloride), and a molten electrolyte, usually sodium chloroaluminate (NaAlCl₄).

An electrically insulating ceramic collar 60, which may be made of alpha-alumina, is situated at a top end 70 of the tube 20. A cathode current collector assembly 80 is disposed in the cathode chamber 50, with a cap structure 90, in the top region of the cell. The ceramic collar 60 is fitted onto the top end 70 of the separator tube 20, and is sealed by a glass seal 100. In one embodiment, the collar 60 includes an upper portion 62, and a lower inner portion 64 that abuts against an inner wall of the tube 20, as illustrated in FIG. 1.

In order to seal the cell 10 at the top end (i.e., its upper region), a metal ring 110 is sometimes disposed. The metal ring 110 has two portions; an outer metal ring 120 and an inner metal ring 130, which are joined, respectively, with the upper portion 62 and the lower portion 64 of the ceramic collar 60, by means of the active braze seals 140 and 150. The active braze seal 140, the seal 150, or both may be formed by using a suitable braze alloy composition described above. The collar 60 and the metal ring 110 may be temporarily held together with an assembly (e.g., a clamp), or by other techniques, until sealing is complete.

The outer metal ring 120 and the inner metal ring 130 are usually welded shut to seal the cell, after joining with the ceramic collar 60 is completed. The outer metal ring 120 can be welded to the cell case 30; and the inner metal ring 130 can be welded to the current collector assembly 80.

The shape and size of the several components discussed above with reference to FIG. 1 are only illustrative for the understanding of the cell structure; and are not meant to limit the scope of the invention. The exact position of the seals and the joined components can vary to some degree. Moreover, each of the terms “collar” and “ring” is meant to comprise metal or ceramic parts of circular or polygonal shape, and in general, all shapes that are compatible with a particular cell design. An additional description of electrochemical cells of this type is provided in pending patent application Ser. No. 13/600,333 (R. Adharapurapu et al), filed on Aug. 31, 2012, the entire contents of which are incorporated herein by reference.

The braze alloys and the active braze seal formed thereof, generally have good stability and chemical resistance within determined parameters at a determined temperature. It is desirable (and in some cases, critical) that the braze seal retains its integrity and properties during several processing steps while manufacturing and using the cell, for example, during a glass-seal process for a ceramic-to-ceramic joint, and during operation of the cell. In some instances, optimum performance of the cell is generally obtained at a temperature greater than about 300° C. In one embodiment, the operating temperature may be in a range from about 270° C. to about 450° C. In one embodiment, the glass-seal process is carried out at a temperature of at least about 1000° C. In some other embodiments, the glass-seal process is carried out in a range of from about 1000° C. to about 1200° C., and in some situations, at even higher temperatures. Moreover, the bond strength and hermeticity of the seal may depend on several parameters, such as the composition of the braze alloy, the thickness of the thin reaction layer, the composition of the ceramic, and the surface properties of the ceramic.

Other inventive embodiments are directed to an energy storage device that includes a plurality of the electrochemical cells as disclosed in previous embodiments. The cells are, directly or indirectly, in thermal and/or electrical communication with each other. Those of ordinary skill in the art are familiar with the general principles of such devices. For example, U.S. Pat. No. 8,110,301 is illustrative, and incorporated by reference herein. However, there are many other references which generally describe various types of energy storage devices, and their construction.

Some embodiments provide a method for joining a first component to a second component by using a braze alloy composition. The method includes the steps of introducing the braze alloy between the first component and the second component to form a brazing structure. (The alloy could be deposited on one or both of the mating surfaces, for example, as also described below). The brazing structure can then be heated to form an active braze seal between the first component and the second component. In one embodiment, the first component includes a ceramic; and the second component includes a metal. (The braze alloy composition is as described previously).

In the general preparation of the braze alloy, a desired alloy powder mixture may be obtained by combining (e.g., mixing and/or milling) commercial metal powders of the constituents in their respective amounts. In some embodiments, the braze alloy may be employed as a foil, a sheet, a ribbon, a preform, or a wire, or may be formulated into a paste containing water and/or organic fluids. In some embodiments, the precursor metals or metal alloys may be melted to form homogeneous melts, before being formed and shaped into particles. In some cases, the molten material can be directly shaped into foils, preforms or wires. Forming the materials into particles, initially, may comprise spraying the alloy melt into a vacuum, or into an inert gas, to obtain a pre-alloyed powder of the braze alloy. In other cases, pellets of the materials may be milled into a desired particle shape and size.

In one embodiment, a layer of the braze alloy is disposed on at least one surface of the first component or the second component to be joined by brazing. The layer of the braze alloy, in a specific embodiment, is disposed on a surface of the ceramic component. The thickness of the alloy layer may be in a range between about 5 microns and about 300 microns. In some specific embodiments, the thickness of the layer ranges from about 10 microns to about 100 microns. The layer may be deposited or applied on one or both of the surfaces to be joined, by any suitable technique, e.g. by a printing process or other dispensing processes. In some instances, the foil, wire, or the preform may be suitably positioned for bonding the surfaces to be joined. In some embodiments, a paste or dispersion of the active metal may initially be applied to a surface of a ceramic component being joined. For example, a layer of titanium paste can be applied in this manner, functioning as a type of primer layer, as described in PCT Application WO 99/65642, incorporated herein by reference.

In some specific embodiments, a sheet or foil of the braze alloy may be desirable. The thickness of the sheets or foils may usually vary between about 20 microns and about 200 microns. The alloys can be rolled into sheets or foils by a suitable technique, for example melt spinning. In one embodiment, the alloy may be melt spun into a sheet or a foil, along with rapid quenching during the spinning.

In a typical embodiment, the method further includes the step of heating the brazing structure at the brazing temperature. When the brazing structure is heated at the brazing temperature, the braze alloy melts and flows over the surfaces. The heating can be undertaken in a controlled atmosphere, such as ultra-high pure argon, hydrogen and argon, ultra-high pure helium; or in a vacuum. To achieve good flow and wetting of the braze alloy, the brazing structure is held at the brazing temperature for a few minutes after melting of the braze alloy, and this period may be referred to as the “brazing time”. During the brazing process, a load can also be applied on the samples.

The brazing temperature and the brazing time may influence the quality of the active braze seal. The brazing temperature is generally less than the melting temperatures of the components to be joined, and higher than the liquidus temperature of the braze alloy. In one embodiment, the brazing temperature ranges from about 900° C. to about 1500° C. , for a time period of about 1 minute to about 30 minutes. In a specific, non-limiting embodiment, the heating is carried out at the brazing temperature from about 1000° C. to about 1300° C., for about 5 minutes to about 15 minutes.

During brazing, the alloy melts, and the active metal element (or elements) present in the melt react with the ceramic and form a thin reaction layer at the interface of the ceramic surface and the braze alloy, as described previously. The thickness of the reaction layer may range from about 0.1 micron to about 2 microns, depending on the amount of the active metal element available to react with the ceramic, and depending on the surface properties of the ceramic component. In a typical sequence, the brazing structure is then subsequently cooled to room temperature; with a resulting, active braze seal between the two components. In some instances, rapid cooling of the brazing structure is permitted.

In some embodiments, an additional layer containing the active metal element may be first applied to the ceramic component. The additional layer may have a high amount of the active metal element, for example more than about 70 weight percent. Suitable examples may include nanoparticles of the active metal element, or a hydride of the active metal element, e.g., titanium hydride.

Some of the embodiments of the present invention advantageously provide braze alloys, which are compositionally stable, and chemically stable in the corrosive environment relative to known braze alloys, and are capable of forming an active braze seal for a ceramic-to-metal joint. These braze alloys have high sodium corrosion resistance, and halide corrosion resistance for many end uses. The formation of ceramic-to-metal seals for high temperature cells (as discussed above) by active brazing simplifies the overall cell-assembly process, and improves the reliability and performance of the cell. The present invention provides advantages to leverage a relatively inexpensive, simple, and rapid process to seal the cell or battery, as compared to currently available methods.

EXAMPLES

The example provided herein is merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention. Unless specified otherwise, all ingredients may be commercially available from such common chemical suppliers as Alpha Aesar, Inc. (Ward Hill, Mass.), Sigma Aldrich (St. Louis, Mo.), Spectrum Chemical Mfg. Corp. (Gardena, Calif.), and the like.

A braze alloy composition of nickel, chromium, niobium, and titanium was prepared, having the nominal composition Ni-27.2Cr-14.1Nb-4Ti (weight percent). In the preparation of the alloy, the individual elements were weighed according to the desired proportions, and then arc-melted to provide an ingot of the material. To ensure homogeneity of the composition, the ingot was triple-melted. The liquidus temperature of the sample was determined to be 1203° C., using a Differential Scanning calorimeter (DSC).

The ingot was formed into approximately a 75 micron-thick sheet, and cooled. The sample sheet was placed between the surfaces of an alpha alumina component and a nickel component to be joined. The assembly was then heated to about 1250° C. for about 10 minutes, and then cooled to room temperature, to form a joint.

FIG. 2 is a cross-sectional SEM image of the brazed components. The image depicts the interface between the braze alloy 200 and the alumina component 202, in the joint region. A reaction layer 204 was observed at the interface, indicating reaction between the braze alloy and the ceramic, and formation of an active braze seal.

The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims. Furthermore, all of the patents, patent applications, articles, and texts which are mentioned above are incorporated herein by reference. 

What is claimed:
 1. A high-temperature thermal battery based on sodium metal halide or sodium sulfur, comprising a sealing region that includes at least one metal component and at least one ceramic component that are joined together by an active braze seal, wherein the seal is formed of a braze alloy composition that comprises a) about 45% to about 70% nickel; b) about 5% to about 40% of at least one refractory metal selected from niobium, tantalum, or molybdenum. c) about 2% to about 32% chromium; and d) about 0.5% to about 10% (total) of at least one active metal element selected from titanium, zirconium, hafnium, and vanadium; based on the weight of the composition.
 2. The high-temperature thermal battery of claim 1, wherein the metal component is a ring, and the ceramic component is a collar, brazed to the ring.
 3. The high-temperature thermal battery of claim 2, further comprising a second ring in the sealing region, brazed to another portion of the collar.
 4. The high-temperature thermal battery of claim 2, wherein the metal ring comprises nickel; and the collar comprises alpha-alumina.
 5. The high-temperature thermal battery of claim 1, wherein the sealing region includes a reaction layer at the interface of the braze alloy and the ceramic component.
 6. The high-temperature thermal battery of claim 5, wherein the reaction layer has a thickness of less than about 10 microns.
 7. The high-temperature thermal battery of claim 1, wherein at least about 50%, by weight, of the refractory metal composition of component (b) is niobium.
 8. The high-temperature thermal battery of claim 7, wherein all of the refractory metal composition of component (b) is niobium.
 9. The high-temperature thermal battery of claim 1, wherein the refractory metal is niobium, present in the braze alloy composition at a level of about 10% to about 40%, by weight.
 10. The high-temperature thermal battery of claim 1, wherein the braze alloy composition comprises about 50% to about 70% nickel.
 11. The high-temperature thermal battery of claim 1, wherein the braze alloy composition has a liquidus temperature of less than about 1250° C.
 12. The high-temperature thermal battery of claim 1, wherein the braze alloy composition is substantially free of iron.
 13. The high-temperature thermal battery of claim 1, wherein the braze alloy composition further comprises aluminum, at a level less than about 5% by weight.
 14. The high-temperature thermal battery of claim 1, wherein the active metal element is present at a level in the range of about 0.5% by weight to about 5% by weight.
 15. The high-temperature thermal battery of claim 1, wherein the active metal element is present at a level in the range of about 1% by weight to about 5% by weight. 